Emergency officials work at the scene where Rep. Gabrielle Giffords, D-Arizona, was shot this morning. She was holding constituent meetings at a Safeway in Tucson when she and 17 others were shot; six have died, including a 9-year-old girl and a federal judge.

The shooting occurred shortly after 10 a.m. local time Saturday as the Arizona lawmaker was holding an outdoor meeting with constituents at a Safeway supermarket. It was billed as her first "Congress on Your Corner" meeting for 2011.

The attack, which injured at least a dozen other people, involved an automatic weapon, according to the Associated Press. Little is known about the suspect, who is in police custody.

Although some early accounts had the congresswoman among the fatally wounded, the Associated Press reports that doctors at the hospital where she was taken say that they are "optimistic" and that she was responding to doctors' instructions.

This past November, Representative Giffords won reelection in a tight race against Republican Jesse Kelly, an Iraq war veteran and favorite of the tea party.

President Obama called the attack "an unspeakable tragedy ... a senseless and terrible act of violence that has no place in a free society."

During a brief statement Saturday afternoon, Mr. Obama said he had sent FBI Director Robert Mueller to Arizona to coordinate the investigation into the attack.

House Speaker John Boehner (R) of Ohio said in a statement, “I am horrified by the senseless attack on Congresswoman Gabrielle Giffords and members of her staff. An attack on one who serves is an attack on all who serve.... This is a sad day for our country."

House minority leader Nancy Pelosi added in a statement, "This terrible act of violence is a national tragedy.... It is especially tragic that she was attacked as she was meeting with her constituents whom she serves with such dedication and distinction."

Voters first sent Giffords, a onetime Republican, to Congress in 2006.

Her husband is NASA astronaut Mark Kelly, who is scheduled to command the final scheduled shuttle mission aboard the orbiter Endeavour later this year.

Following Giffords's vote in favor of Obama's health-care reform package, the lawmaker's Tucson office was vandalized. The attack was widely seen as part of a broader backlash by more-radical elements of the tea party against lawmakers who back the health-care overhaul.